How the maths actually works

Combining five selections at even money returns 32 times the stake. It also requires all five to land. At a genuine 50% chance each, that happens about 3% of the time.

• Two legs — roughly one in four
• Three legs — roughly one in eight
• Five legs — roughly one in thirty-two
• Each added leg cuts the strike rate sharply

Where opening lines drift

Early prices are set before most of the market has an opinion. They move as money arrives, so a line that looks generous on Tuesday may not exist on Saturday.

Team news is the usual culprit

A confirmed absence moves a line further than almost anything else, and it lands at a predictable time — usually an hour before kick-off.

The value is not in picking winners. It is in taking a price before the rest of the market agrees with you.

A sane way to build one

1. Price each leg on its own before looking at the combined return
2. Drop any leg you would not back as a single
3. Keep the number of legs low enough that the strike rate stays real
4. Decide the stake before seeing the potential return, not after

The short version

Accumulators are entertainment with a long tail. Treat the headline return as the least interesting number on the slip.

The three formats

• Decimal — total return per unit staked, including the stake
• Fractional — profit relative to stake, excluding it
• American — stake needed to win 100, or profit from staking 100

Decimal makes the arithmetic obvious

Odds of 2.50 return two and a half times the stake in total. Divide one by the decimal odds and you get the implied probability directly — 1 / 2.50 is 0.40, or 40%.

Implied probability is the only number that lets you compare a price against your own opinion.

Why the probabilities sum above 100%

Add the implied probabilities of every outcome and the total exceeds one hundred percent. The excess is the margin built into the market — the cost of taking the bet.

Reading a price in practice

1. Convert to decimal if it is not already
2. Divide one by the odds for implied probability
3. Compare that against your own estimate
4. Add up the whole market to see the margin you are paying

The short version

Pick one format and stay with it. Decimal makes implied probability a single division, which is the calculation that actually matters.